Chapter 7 Deadlocks

1. Bilkent University Department of Computer Engineering CS342 Operating Systems Chapter 7Deadlocks Dr. Selim Aksoy http://www.cs.bilkent.edu.tr/~saksoy Slides courtesy of Dr. İbrahim Körpeoğlu

2. Outline The Deadlock Problem System Model Deadlock Characterization Methods for Handling Deadlocks Deadlock Prevention Deadlock Avoidance Deadlock Detection Recovery from Deadlock Objectives To develop a description of deadlocks, which prevent sets of concurrent processes from completing their tasks To present a number of different methods for preventing or avoiding deadlocks in a computer system Objectives and Outline

3. The Deadlock Problem • A set of blocked processes each holding a resource and waiting to acquire a resource held by another process in the set • Example • System has 2 disk drives • P1 and P2 each hold one disk drive and each needs another one • Example • semaphores A and B, initialized to 1 P0P1 wait (A); wait(B) wait (B); wait(A)

4. Bridge Crossing Example • Traffic only in one direction • Each section of a bridge can be viewed as a resource • If a deadlock occurs, it can be resolved if one car backs up (preempt resources and rollback) • Several cars may have to be backed up if a deadlock occurs • Starvation is possible • Note – Most OSes do not prevent or deal with deadlocks section1 section2

5. System Model • Resource types R1, R2, . . ., Rm CPU cycles, memory space, I/O devices • Each resource type Ri has Wi instances. • Each process utilizes a resource as follows: • request (may cause the process to wait) • use • release

6. Deadlock Characterization • Deadlocks can arise if four conditions hold simultaneously • Mutual exclusion: only one process at a time can use a resource • Hold and wait: a process holding at least one resource is waiting to acquire additional resources held by other processes • No preemption: a resource can be released only voluntarily by the process holding it, after that process has completed its task • Circular wait: there exists a set {P0, P1, …,PN, P0} of waiting processes such that P0 is waiting for a resource that is held by P1, P1 is waiting for a resource that is held by P2, …, Pn–1 is waiting for a resource that is held by Pn, and Pn is waiting for a resource that is held by P0.

7. Resource-Allocation Graph A set of vertices V and a set of edges E. • V is partitioned into two types: • P = {P1, P2, …, Pn}, the set consisting of all the processes in the system • R = {R1, R2, …, Rm}, the set consisting of all resource types in the system • request edge– directed edge Pi  Rj • assignment edge– directed edge Rj  Pi

8. Resource-Allocation Graph (Cont.) • Process • Resource Type with 4 instances • Pirequests instance of Rj • Pi is holding an instance of Rj Pi Rj Pi Rj

9. Example of a Resource Allocation Graph

10. Resource Allocation Graph With A Deadlock There is a cycle and Deadlock

11. Graph With A Cycle But No Deadlock There is a cycle but No Deadlock

12. Basic Facts • If graph contains no cycles  no deadlock • If graph contains a cycle  • if only one instance per resource type, then deadlock • if several instances per resource type, possibility of deadlock

13. Methods for Handling Deadlocks • Ensure that the system will never enter a deadlock state • Deadlock Prevention or Deadlock Avoidance methods • Allow the system to enter a deadlock state and then recover • Deadlock Detection needed • Ignore the problem and pretend that deadlocks never occur in the system; • Used by most operating systems, including UNIX • OS does not bother with deadlocks that can occur in applications

14. Deadlock Prevention • Mutual Exclusion – not required for sharable resources; must hold for nonsharable resources • Hold and Wait – must guarantee that whenever a process requests a resource, it does not hold any other resources • Require process to request and be allocated all its resources before it begins execution, or allow process to request resources only when the process has none • Low resource utilization; starvation possible Basic Principle: Restrain the ways requests can be made

15. Deadlock Prevention (Cont.) • No Preemption – • A processing holding resources make requests: if request cannot be granted, release (preempt) the held resources, and try again later. • Preempted resources are added to the list of resources for which the process is waiting • Process will be restarted only when it can regain its old resources, as well as the new ones that it is requesting. • Circular Wait – impose a total ordering of all resource types, and require that each process requests resources in an increasing order of enumeration

16. Deadlock Prevention (Cont.) • All resources are ordered and assigned an integer number • A process can request resources in increasing order of enumeration Resources R5 R1 R2 R3 R4 can only request and allocate in this order Example: Process 2 Request R1 Request R2 Request R3 Process 3 Request R3 Request R4 Process 1 Request R2 Request R4

17. Proof • Consider the resources that are allocated at the moment. Consider the process that has the highest numbered allocated resource. • That process will not block; will be able to continue and finish. Because: • It can not make a request to a resource with a smaller number and get block. This will not happen. • It can make a request to a resource with a larger number. That resource is not allocated yet (otherwise that would be the highest numbered allocated resource). Hence the process will get the resource immediately. In this way, that process will not block. Will be able to run and complete. • Then, the same thing will be applicable to the process that is holding the next highest numbered resource. That will be able to run an finish. • All process may run and finish sooner or later.

18. Deadlock Avoidance Basic Principle: Requires that the system has some additional a priori information available • Simplest and most useful model requires that each process declarethe maximum number of resources of each type that it may needto hold simultaneously. (maximum demand) • The deadlock-avoidance algorithm dynamically examines the resource-allocation state to ensure that there can never be a circular-wait condition. • Resource-allocation state is defined by the number of available and allocated resources, and the maximum demands of the processes

19. Safe state • When a process requests an available resource, system must decide if immediate allocation leaves the system in a safe state • A state is safe if the system can allocate resources to each process (up to its maximum) in some order and still avoid a deadlock. • We are considering a worst-case situation here. Even in the worst case (process requests up their maximum at the moment), we don’t have deadlock in a safe state.

20. Safe state • More formally: A system state is safeif there exists a safe sequence of all processes (<P1, P2, …, Pn>) such that for each Pi, the resources that Pi can still request can be satisfied by currently available resources + resources held by all the Pj, with j < i • That is: • If Pi resource needs are not immediately available, then Pi can wait until all Pjhave finished • When Pj is finished, Pi can obtain needed resources, execute, return allocated resources, and terminate • When Pi terminates, Pi +1 can obtain its needed resources, and so on.

21. Basic Facts • If a system is in safe state  no deadlocks • If a system is in unsafe state  possibility of deadlock • Avoidance  ensure that a system will never enter an unsafe state. • When a request is done by a process for some resource(s): check before allocating resource(s); if it will leave the system in an unsafe state, then do not allocate the resource(s); process is waited and resources are not allocated to that process.

22. Safe, Unsafe , Deadlock State

23. Avoidance Algorithms • Single instance of a resource type • Use a resource-allocation graph • Multiple instances of a resource type • Use the banker’s algorithm

24. Resource-Allocation Graph Scheme • Claim edgePi Rj indicates that process Pimay request resource Rj; represented by a dashed line • Claim edge is converted to a request edge when a process requests a resource • Request edge is converted to an assignment edge when the resource is allocated to the process • When a resource is released by a process, assignment edge is reconverted to a claim edge • Resources must be claimed a priori in the system

25. Resource-Allocation Graph

26. Resource-Allocation Graph P2 requests R2; should we allocate?

27. Unsafe State In Resource-Allocation Graph

28. Resource-Allocation Graph Algorithm • Suppose that process Pi requests a resource Rj • The request can be granted only if converting the request edge to an assignment edge does not result in the formation of a cycle in the resource allocation graph.

29. Banker’s Algorithm • Multiple instances • Each process must a priori claim maximum use • When a process requests a resource it may have to wait • When a process gets all its resources it must return them in a finite amount of time

30. Simple Example • Assume a resource A (printer) has 5 instances. • There are 2 processes: P1, P2 • Max demand is: 5 5 • Current Allocation is: 3 0 • Need is: ….. (write on the board) • Available is: ….. Is the current state safe? Solve the problem on the board

31. Simple Example • Assume P2 request 1 instance and it is granted. Is the new state safe? • Current Allocation will be : 3 1 • Need is: ….. (write on the board) • Available is: ….. Solve the problem on the board

32. Data Structures for the Banker’s Algorithm Let n = number of processes, and m = number of resources types. • Available: Vector of length m. If Available[j] == k, there are k instances of resource type Rjat the time deadlock avoidance algorithms is run. • Max: n x m matrix. If Max[i,j] == k, then process Pimay request at most k instances of resource type Rj • Allocation: n x m matrix. If Allocation[i,j] == k then Pi is currently allocated k instances of Rj • Need: n x m matrix. If Need[i,j] = k, then Pi may need k more instances of Rjto complete its taskNeed[i,j] = Max[i,j] – Allocation[i,j]

33. An example system state Initially Available == Existing Allresourcesin the system system state at some t (may change) Need = Max - Allocation

34. Notation Compare two vectors: X is a matrix. Xi is the ith row of the matrix: it is a vector. For example, X3 = [2 1 1] Ex: compare V with Xi V Xi V == Xi ? V <= Xi ?Xi <= V ? …. V is a vector; V = [3 3 2] Ex: Compare [3 3 2] with [2 2 1] [2 2 1] <= [3 3 2]

35. Safety Algorithm 1. Let Workand Finishbe vectors of length m and n, respectively. Initialize: Work = Available (initialize Work temporary vector) Finish [i] = false for i = 0, 1, …, n-1 (Work is a temporary vector initialized to the Available (i.e., free) resources at that time when the safety check is performed) 2. Find an i such that both: (a) Finish [i] = false (b) Needi Work If no such i exists, go to step 4 • Work = Work + AllocationiFinish[i] = truego to step 2 4. If Finish [i] == true for all i, then the system state is safe; o.w. unsafe. Available [3 3 2]

36. Resource-Request Algorithm for Process Pi Request: request vector for process Pi. If Requesti[j] == k, then process Piwants k instances of resource type Rj Algorithm 1. If Requesti  Needigo to step 2. Otherwise, raise error condition, since process has exceeded its maximum claim 2. If Requesti Available, go to step 3. Otherwise Pi must wait, since resources are not available

37. Resource-Request Algorithm for Process Pi • 3. Pretend to allocate requested resources to Pi by modifying the state as follows: Available = Available – Requesti; Allocationi = Allocationi + Requesti; Needi= Needi– Requesti; Run the Safety Check Algorithm: • If safe  the requested resources are allocated to Pi • If unsafe  The requested resources are not allocated to Pi. Pi must wait. The old resource-allocation state is restored.

38. Example of Banker’s Algorithm • 5 processes P0 through P4; • 3 resource types: A, B, and C • Existing Resources: A (10 instances), B (5 instances), and C (7 instances) • Existing = [10, 5, 7] initially, Available = Existing. • Assume, processes indicated their maximum demand as follows: Initially, Allocation matrix will be all zeros. Need matrix will be equal to the Max matrix.

39. Example of Banker’s Algorithm • Assume later, at an arbitrary time t, we have the following system state: Need = Max - Allocation Existing = [10 5 7] Is it a safe state?

40. Example of Banker’s Algorithm Try to find a row in Needi that is <= Available. P1. run completion. Available becomes = [3 3 2] + [2 0 0] = [5 3 2] P3. run completion. Available becomes = [5 3 2] + [2 1 1] = [7 4 3] P4. run completion. Available becomes = [7 4 3] + [0 0 2] = [7 4 5] P2. run completion. Available becomes = [7 4 5] + [3 0 2] = [10 4 7] P0. run completion. Available becomes = [10 4 7] + [0 1 0] = [10 5 7] We found a sequence of execution: P1, P3, P4, P2, P0. State is safe

41. Example: P1 requests (1,0,2) • At that time Available is [3 3 2] • First check that Request  Available (that is, (1,0,2)  (3,3,2)  true. • Then check the new state for safety: new state (we did not go to that state yet; we are just checking)

42. Example: P1 requests (1,0,2) new state Can we find a sequence? Run P1. Available becomes = [5 3 2] Run P3. Available becomes = [7 4 3] Run P4. Available becomes = [7 4 5] Run P0. Available becomes = [7 5 5] Run P2. Available becomes = [10 5 7] Sequence is: P1, P3, P4, P0, P2 Yes, New State is safe. We can grant the request. Allocate desired resourcesto process P1.

43. P4 requests (3,3,0)? Current state If this is current state, what happens if P4 requests (3 3 0)? There is no available resource to satisfy the request. P4 will be waited.

44. P0 requests (0,2,0)? Should we grant? Current state System is in this state. P0 makes a request: [0, 2, 0]. Should we grant.

45. P0 requests (0,2,0)? Should we grant? Assume we allocate 0,2,0 to P0. The new state will be as follows. New state Is it safe? No process has a row in Need matrix that is less than or equal to Available. Therefore, the new state would be UNSAFE. Hence we should not go to the new state. The request is not granted. P0 is waited.

46. Deadlock Detection • Allow system to enter deadlock state • Detection algorithm • Recovery scheme

47. Single Instance of Each Resource Type • Maintain wait-for graph • Nodes are processes • Pi Pj if Piis waiting for Pj • Periodically invoke an algorithm that searches for a cycle in the graph. If there is a cycle, there exists a deadlock • An algorithm to detect a cycle in a graph requires an order of n2 operations, where n is the number of vertices in the graph

48. Single Instance of Each Resource Type

49. Several Instances of Resource Type • Available: A vector of length m indicates the number of available resources of each type. • Allocation: An n x m matrix defines the number of resources of each type currently allocated to each process. • Request: An n x m matrix indicates the current request of each process. If Request [ij] = k, then process Pi is requesting k more instances of resource type. Rj. System state is represented by this information

50. Detection Algorithm 1. Let Work and Finish be vectors of length m and n, respectively. Initialize: (a) Work = Available (b) For i = 1,2, …, n, if Allocationi 0, then Finish[i] = false; otherwise, Finish[i] = true 2. Find an index i such that both: (a) Finish[i] == false (b) Requesti Work If no such i exists, go to step 4